Biosensors are devices that allow qualitative or quantitative detection of target molecules, also called “analytes”, such as e.g. proteins, viruses, bacteria, cell components, cell membranes, spores, DNA, RNA, etc. in a sample fluid comprising for example blood, serum, plasma, saliva, tissue extract, intestinal fluid, cell culture extract, food or feed extract, drinking water, etc. Often a biosensor uses a sensor surface that comprises specific recognition elements for capturing the analyte. The surface of the biosensor device may therefore be modified by attaching specific molecules to it, which are suitable to bind the target molecules to be detected in the sample fluid. A well established principle is the counting of labelled molecules of interest captured at predetermined sites on the biosensor. For example, such molecules of interest may be labelled with magnetic particles or beads and these magnetic particles or beads can be detected with a magnetic sensor. One alternative is detection of the amount of analyte using optical detection such as fluorescence. In this case, the analyte itself may carry a fluorescent label, or alternatively an additional incubation with a fluorescent labelled recognition element may be performed.
In most biosensors, the sensor chip is provided with a dry reagent in addition to the sensor surface. The reagent may e.g. comprise labels coupled to biologically-active moeities, e.g. an anti-drug antibody. In order to limit the analysis time, the reagent can be deposited directly on the sensor surface. When the test fluid arrives, the dry reagent dissolves and mixes into the fluid which will then wet the sensor surface. The labels as well as the sensor surface are exposed to the target, (e.g. drug) molecules. This influences the binding of the labels to the sensor surface, which is detected. An inconvenience of having the reagent deposited directly on the sensor surface is that it leads to possible premature reaction or mixing of the reagent with the sensor surface (i.e. before the reagent has had the possibility to react with the target), thus disturbing the detection.
A bio-sensing system wherein the reagent is physically separated from the sensor surface is disclosed in Fukumoto et al (The 13th International Conference on Solid-State Sensors, Actuators and Microsystems, Seoul, Korea, Jun. 5-9, 2005). In this article, a test cartridge is disclosed comprising a detection chamber equipped with a sensor chip, on which a capture antibody is immobilised, and a cap in which a sample-loading hole is performed. A non-woven fabric, on which detection antibody bound magnetic particles are dotted and freeze-dried, is fixed to the cap in such a way as to cover the hole. A sample including an antigen is then dropped on the cartridge.